It is well known to use light-emitting elements, such as LED's, in printing devices, see U.S. Pat. No. 4,780,731 and European Patent Application A-0189664. Typically, two rows of LED's are used to achieve a uniform exposure, see NL-A 8300111. However, using two rows requires specialized synchronization between the data for the first and second rows.
The use of a single row of light-emitting elements has resulted in nonuniform exposures. See, for example, Japanese Application 60-99672, 63-309476 and 63-57262.
In U.S. Pat. No. 4,553,148, there is disclosed an exposure device comprising one straight row of light-emitting elements disposed with fixed spacing b between them in one plane. In this case the LED's are disposed in a straight row to image, by means of a Selfoc array, onto a moving photoconductor. In order to increase the light yield, the LED's are constructed in the form of elongate rectangles or parallelograms. By disposing a cylindrical lens between the row of LED's and the photoconductor, the elongate LED's are imaged as small squares or diamonds.
A disadvantage of these known devices is that the light distribution on the photosensitive medium as considered in the direction of the row is not equal, and between two image dots there is a zone where there is distinctly less light. To enable the photoconductor to be exposed at these places too, the total light level has to be increased e.g., by increasing the LED's energization current, and this increases the heat development and reduces the LED life.
Accordingly, it is an object of the present invention to provide a means for achieving a uniform exposure using a single row of light-emitting elements.